1. Field of the Invention
The invention relates to a cylinder block for an internal-combustion engine, having bevels arranged at the cylinder bores for insertion of the cylinders.
2. Background Information
Since, in the uncompressed state, the piston rings have a larger diameter than the cylinder bore, the rings have to be compressed during the installation of the piston and have to be introduced axially into the cylinder in the compressed state. To ensure that the bearing surface of the cylinder bore and the piston rings are damaged as little as possible, compressors are fitted onto the top side (facing the cylinder head) of the cylinder block and are centered by means of a bevel arranged on the cylinder bore.
DE 39 20 845-C1 discloses a mounting tool for inserting pistons that are provided with piston rings into a cylinder. The tool is fitted onto the bevel of the cylinder bore. Making the bevel on the upper side of the cylinder is highly disadvantageous, since the bevel itself and the cavity which surrounds it between cylinder wall/piston ring/piston, on the one hand, and the gasket and cylinder head, on the other hand, represents a dead space in terms of flow, and the combustion in these areas only takes place incompletely. This leads to increased emissions of hydrocarbons.
According to the invention, it is provided that part of the crankshaft-side end of the cylinder bore is designed as a run-up cone (31) as shown in FIG. 3.
This inventive measure provides numerous advantages. Firstly, the bevel on the upper side of the cylinder block can be dispensed, or at most the cylinder edge may be slightly tapered. Consequently, the gasket can be placed closer to the cylinder bore wall, and accordingly the diameter of the combustion chamber in the cylinder head at this location can be brought closer to the cylinder bore wall. This reduces the dead space volume and considerably improves the raw emissions from the engine. Since the gasket between the cylinder bores must have a certain minimum web width, so that the sealing function is adequately ensured, and the gasket can now be positioned closer to the cylinder bore wall, overall the cylinders can be brought closer together. This allows the overall length of the engine to be reduced.
Moreover, there are manufacturing technology advantages. The bevel, which was previously arranged on the upper side of the cylinder block, always represented a stepped transition with respect to the installation tool. The piston rings were often damaged at this transition, and this damage is now avoided. Moreover, installation is simplified considerably. There is no need to fit and align the installation tool for each individual piston. Furthermore, unlike with a compressor, the piston rings are uniformly compressed over their circumference as a result of the introduction over the run-up cone. This also avoids damage to the piston rings. Finally, the engine is easier to repair, since pistons and piston rings can be fitted securely, if appropriate even after machining of the cylinder bearing surface.
The run-up cone may in particular have not just one included angle (xcex4) over its height h, but rather a plurality of such angles. In particular, the run-up cone may, from its lower crankshaft-side end toward the cylinder bearing surface, have a curved profile. In this case, a preferred form may include the cone having, at its lower end, a cylindrical attachment, with the result that the included angle of the run-up cone over the height increases from a value of close to zero to a maximum value (turning point) and then decreases back to close to zero toward the cylinder bearing surface. In this way, the risk of damage to the piston rings can be reduced still further.
Preferably, the height of the run-up cone in the axial direction of the cylinder bore is at least equal to the maximum distance of the uppermost piston ring from the piston top surface (measured at the outer circumference of the latter if the top surface is not planar).
Since the opening of the run-up cone at its lower end (B1) is greater than the maximum diameter of all the piston ringsxe2x80x94even if the piston rings are offset in the radial direction with respect to one another and/or with respect to the piston axisxe2x80x94the piston can be mounted reliably. The set minimum height ensures that the piston itself is initially guided in the run-up cone before the first piston ring enters the area of the lower opening of the run-up cone. This prevents the upper piston ring from jamming against the lower edge of the run-up cone.
It is also preferable for the maximum value of the included angle (xcex4) of the run-up cone not to exceed 45xc2x0. Setting the included angle of the run-up cone further improves the ease of installation of the pistons.
Further advantages, objects and features of the invention will become apparent from the following detailed description of the invention taken in conjunction with the accompanying figures showing illustrative embodiments of the invention.